This disclosure relates to the field of pulping machinery for defibering papermaking stock, and more particularly, to an improved mixing impeller.
The traditional pulper impeller has two separate functions, first, defibering the stock suspension and second, efficient circulation of the suspension to keep it homogeneous. While pulping impellers of various forms have been described in the prior art, such impellers normally represent a compromise between efficient defibering and efficient circulation.
FIG. 1 shows a conventional pulping, mixing, or defibering apparatus, which generally includes a vat, or tub, 10 formed of side wall 11 and bottom wall 12. In the center of the bottom wall 12 is a perforated strainer grate 13. The strainer grate 13 permits draining of pulped paper stock, for example, after a pulping operation is completed. An impeller 15 for circulating the paper stock, for example, or other material, is mounted on a hub 14 in the center of the strainer grate 13. Supports 19 stabilize the pulping tub, or vat, 10.
The impeller 15 creates a mechanical shear and/or hydraulic shear effect on the pulp, or other material, being mixed. Mechanical shear, for example, is achieved by rotating the impeller 15 above and in close proximity to the stationary strainer grate 13 so that the paper pulp stock, or other material, is defibered or separated into individual fibers as a result of the shearing action between the strainer grate 13 and the underside of the vanes 17 of the impeller 15. Hydraulic shear, on the other hand, occurs by contacting the paper pulp fibers, for example, with other paper pulp fibers in the tub, or vat, 10 as a result of the turbulence, or flow pattern, generated by rotation of the impeller 15. Gears that engage the hub 14 drive the impeller 15. A motor 22 powers the gears that are housed within gear housing 20.